Road surfaces (known as pavements) are routinely constructed from asphalt mixture, which is a composite of binder, aggregate and filler (mineral powder). For most pavements, the binder is bitumen, a viscous liquid or a solid consisting essentially of hydrocarbons and their derivatives. The asphalt mixture is prepared in an asphalt mix plant, having facilities for heating and mixing the components. It is necessary to heat aggregate to remove any moisture and to raise it to a temperature suitable for coating with the binder. The removal of moisture is necessary to allow adhesion of the binder to the aggregate. An aggregate dryer/heater typically consists of a large rotating metal drum, heated by a gas or oil burner. The drum is mounted at a slight incline, and the aggregate is dried and heated as it passes through the drum. The heated aggregate is then mixed with binder in a mixing zone, e.g. in a pug mill in a batch process or in the mixing zone of a drum mixer in a continuous process.
It has become increasingly common to incorporate recycled asphalt pavement (RAP) into asphalt mixtures. The RAP is reclaimed from old road surfaces and consists of aggregate and aged binder. During its service life, the binder contained in upper road surfaces is subject to oxidation due to exposure to oxygen in the atmosphere and to UV light. The aged binder is harder and causes pavement embrittlement. The chemical nature of the binder can be characterized by four different fractions: saturates, aromatics, resins and asphaltenes. The changes in the chemical nature of the binder can be assessed by separation techniques such as a combination of heptane precipitation and High Pressure Liquid Chromatography (HPLC) or by Clay Gel Analysis (ASTM D 2007). During the ageing process, the asphaltene and resin contents increase whereas the saturate and aromatic content decrease.
The RAP is typically crushed and sieved before it is recycled. It is combined with virgin aggregate, with filler and with new binder to prepare an asphalt mixture. Up to 25 wt % RAP (based upon the weight of the asphalt mixture) may be incorporated simply by adding the RAP directly to the mixer of an asphalt mix plant. The RAP is not pre-heated, so the virgin aggregate is overheated such that a temperature exchange can occur in the mixer whereby the RAP is heated to the required temperature.
It is desirable to incorporate higher percentages of RAP into asphalt mixtures because it is a more sustainable and lower cost product than virgin aggregate. To incorporate greater than 25 wt % RAP, indirect heating is insufficient to bring the mixture to the desired temperature. Therefore it is typical to pre-heat the RAP in a drum that is especially designed for heating RAP. This drum, usually known as the parallel drum, typically consists of an inclined steel drum with a burner mounted at the higher end. The RAP is dried and heated to a temperature of about 110-130° C. Higher temperatures are not used because of the risk that the binder in the RAP will oxidise and harden. It is usually still necessary to overheat the virgin aggregate such that the RAP can be further heated by temperature exchange in the mixer. It is possible to achieve the required mixing temperatures with a greater quantity of RAP, e.g. up to about 60 wt %.
It is important to ensure that the RAP is entirely broken down into its components parts during the mixing phase. If the RAP is not fully broken down, the resulting asphalt mixture may not be sufficiently homogeneous and the mechanical properties of the resulting asphalt pavement may suffer. The European norm EN 13108-7 on porous asphalt states that the use of RAP should not exceed 10% in top layers and 20% in lower layers unless the customer and the mix producer have sufficient know-how to produce durable asphalt mixtures. Maintaining high temperatures is one way to encourage disintegration of the RAP, but it is desirable to reduce the temperature of the asphalt mix preparation for environmental reasons and to reduce energy consumption and costs.
DE 10 2004 055 474 discloses a method for producing a bituminous composition for road construction, wherein the composition incorporates RAP. An additive such as flux oil is added to RAP to facilitate the processing of the RAP and the resulting asphalt production.
It has also become increasingly desirable to produce asphalt mixtures by so-called warm mix processes wherein the components are not heated above 130° C. Warm mix processes are described in EP 863 949, EP 977 813 and EP 1 263 885. Warm mix processes have lower energy requirements than conventional hot mix processes and therefore have environmental benefits. The prior art describes warm mix processes wherein a soft binder material is mixed with aggregate, and then a hard binder material, possibly in the form of an emulsion or a foam, is added to the mixture of soft binder and aggregate. It has proved difficult to use RAP in warm mix processes because at the warm temperatures there can be insufficient disintegration of the RAP, leading to inhomogeneous asphalt mixtures.
The present inventors have sought to develop improved warm mix processes for preparing durable asphalt mixtures, wherein RAP can be incorporated into the asphalt.